Push Down Field

Push Down Field moves a field from a base class into the subclasses where it belongs. It removes irrelevant state from other subtypes and makes the data model more honest. 1

Original version: every character has mana

In this particular game, mages spend mana and warriors do not. The base class nevertheless gives both a mana value.

public abstract class Character
{
    public string Name { get; } = "character";
    public int Mana { get; set; } = 10;
}
public sealed class Mage : Character { }
public sealed class Warrior : Character { }

class Character:
    def __init__(self):
        self.name = "character"
        self.mana = 10


class Mage(Character):
    pass


class Warrior(Character):
    pass

The warrior carries a field that has no defined role. Other code may reasonably assume that a public mana value means every character can participate in mana-related operations.

Revised version: only the mage stores mana

public abstract class Character
{
    public string Name { get; } = "character";
}
public sealed class Mage : Character
{
    public int Mana { get; set; } = 10;
}
public sealed class Warrior : Character { }

class Character:
    def __init__(self):
        self.name = "character"


class Mage(Character):
    def __init__(self):
        super().__init__()
        self.mana = 10


class Warrior(Character):
    pass

The mage’s initial value remains ten. A warrior now contains only the common character state shown here.

Move the behavior that needs the field too

If base-class methods still read mana, moving only the field leaves those methods with a missing dependency. Move the relevant methods to a suitable capability, introduce a focused interface, or reconsider the hierarchy.

This change removes an observable property from warriors and from the base type. Inspect serialization, saved-game formats, bindings, and clients typed as the base class before adopting it. Preserving mage behavior does not make every old access compatible.

If many independent capabilities can appear on characters, composition may avoid an expanding inheritance tree: a character can have a mana component without being forced into a single rigid category. Push Down Field is useful when the existing hierarchy accurately expresses the distinction, not when it merely postpones a larger modeling problem.

Sources

1. Martin Fowler — Push Down Field.